When light propagates through a uniform, homogeneous medium, it travels in straight lines. However, when inhomogeneities are introduced into the propagation medium, light is scattered by these inhomogeneities, with some scattering occurring in all directions. Light scattered from inhomogeneities in the medium which reach the receiver are a noise source, as this light does not contain information about a scene (including a specified object) which the observer desires to view. As such, it represents a signal which tends to lower the available scene contrast from what it would be without the scattering. The scattering is a function of the type, size (size distribution), material, and spatial distribution of the scattering inhomogeneities. In a water medium such as saltwater, this spatial distribution can change on a rapid (μs) basis.
Previous approaches to achieving differential imaging fall into two broad categories:
(1) using a single camera, with two successive light pulses being used to obtain the two signal returns; and,
(2) using two cameras with gates temporally offset. Both approaches have substantial drawbacks, as indicated below.
Using a single camera imaging successive light pulses at different gate delays relies on the spatial stability of the medium. In most cases, a relatively low repetition rate laser is used as the illumination source. One example of this type of a system is disclosed in U.S. Pat. No. 5,631,704 to Dickinson et al., issued May 20, 1997, and incorporated herein by reference.
Typically, a 30 Hz laser is used, giving a time delay between return (1) and return (2) on the order of 33 ms. In such an arrangement, the exact distribution of scatterers in the medium has changed significantly during the subject time frame, thereby resulting in a scattering pattern which is no longer identical. Even if the magnitudes of the scatterings are similar, the detailed patterns are usually different. Such a scheme may be acceptable provided the camera is effectively stationary over the time between gathering the two returns; it is unacceptable for a moving camera.
Using the two camera approach (described in more detail in U.S. Pat. No. 4,862,257 to Ulich, dated Aug. 29, 1989 and incorporated herein by reference), a different set of problems is encountered. First, since the differential imaging must be achieved on a pixel-to-pixel basis, it is necessary to accurately align the focal planes between the cameras. Second, parallax due to the spatial separation between the two focal planes must be eliminated. Third, accurate timing synchronization between the two cameras is required. In principal, these factors can be successfully adjusted. However, when the assembly is allowed to move, vibratory and displacement effects ensure misalignment between the two cameras, making the differential image unsatisfactory.
While the conventional art systems are satisfactory under certain conditions, they are not entirely suitable in all environments, or applications (such as a moving camera). For example, carrying out mine sweeping operations in agitated or otherwise murky saltwater under harsh weather conditions would render accurate imaging of conventional systems highly problematical. Consequently, there is a need for accurate imaging under extreme environmental conditions for detecting objects in high dispersion or light scattering mediums such as saltwater.